Conventional mass spectrometers, such as linear quadrupole mass spectrometers, have been widely used for many years. Linear quadrupole mass spectrometers use four parallel spaced hyperbolic surfaces with appropriate voltages to establish a two-dimensional quadrupole field. A popular close approximation to the hyperbolic surfaces uses four parallel spaced round rods. Such mass spectrometers act as a filter, transmitting ions in a selected range of mass to charge ratios when the ions are injected into one end of the elongated space between the rods. Such mass spectrometers have performed well. However their full scan sensitivity (defined as number of ions detected/given amount of sample injected) is low, since during scanning they transmit ions of only one mass at a time, and during such transmission, ions of all other masses from the source are wasted.
Examples of linear mass spectrometers are those shown (for example) in U.S. Pat. Nos. 4,329,582; 5,248,875, and 4,963,736.
Partly because of the low sensitivity (as defined above) of conventional linear mass spectrometers, mass spectrometers known as ion traps have become more popular. Ion traps utilize a ring electrode and a pair of end caps, all of which have hyperbolic surfaces, with appropriate voltages to establish a three-dimensional trapping field which traps ions within a mass range of interest in the relatively small volume between the ring electrode and end caps. Various potentials may then be applied to eject ions (usually sequentially) for analysis. If the time needed to manipulate the ions in the ion trap and to scan them out of the trap is small in relation to the time needed to fill the trap, then fewer ions are wasted by the trap and hence the efficiency of the trap can be higher than that of a linear mass spectrometer.
Examples of patents which show ion traps are U.S. Pat. No. 4,540,884, U.S. Pat. No. Re. 34,000, and U.S. Pat. No. 5,381,007.
While ion traps tend to be more sensitive than linear mass spectrometers, ion traps suffer from several disadvantages. One disadvantage is that because the trapping volume of a conventional ion trap is relatively small, the number of ions which it can accept before space charge effects in the trap volume create a serious problem is quite limited (typically an ion trap can accept a maximum of only about one million ions). Since it can accept so few ions, many ions from the sample may again be wasted, resulting in relatively low sensitivity.
In addition, because space charge effects can create a non-linear response, the dynamic range of a trap (i.e. the range over which the response remains linear with respect to the injected sample) is limited. Further, it is commonly necessary to conduct a pre-check before using the ion trap for analysis, to determine if there is a space charge problem.
Another disadvantage of a conventional ion trap is that more than 90% of externally created ions injected into the trap are lost, principally due to the small trap volume (many of the ions entering the trap impact a neutralizing surface and are lost). Typically only 3% to 10% of the ions entering the trap are in fact trapped.
It would therefore be desirable to create an improved mass spectrometer which has at least some of the advantages of an ion trap, e.g. a shorter time to manipulate and scan out all the ions of interest, and yet which overcomes at least some of the disadvantages.